Recent years have seen increasing consideration of so-called fly-by-wire aircraft operating systems. In fly-by-wire aircraft, the aircraft control surfaces are not mechanically and/or hydraulically linked to the controls operated by the pilot or by an automatic pilot. Rather, the controls in the control surfaces are linked electrically.
Consequently, in order to maintain control over a fly-by-wire aircraft, it is necessary that electric power be present at all times during flight. Since there is always the possibility that an electrical generator driven by a main propulsion engine might fail for any of a variety of reasons, most fly-by-wire aircraft systems include provision for one or more power generating units that is not operated by the main propulsion engine or engines, but rather, by a separate turbine. Typically, such power units are in the form of auxiliary power units (APU), emergency power units (EPU), or both, or a combined auxiliary-emergency power unit.
In any event, where electrical power is lost, it is necessary that it be restored within two to three seconds to enable control over the aircraft to be regotten. This means that the source of electrical power during the emergency, whether an APU or and EPU or a combined APU/EPU must be started and brought up to operating speed extremely rapidly.
While under many conditions, this is not a particularly difficult chore, as altitude increases, the thinness of the air and the resultant scarcity of oxygen on a mass basis make it not only difficult to start the turbine engine of the power unit, but difficult to accelerate it to full operating speed as well.
The present invention is directed to overcoming one or more of the above problems.